Method for forming accumulation-mode field effect transistor with improved current capability

ABSTRACT

A method of forming an accumulation-mode field effect transistor includes forming a channel region of a first conductivity type in a semiconductor region of the first conductivity type. The channel region may extend from a top surface of the semiconductor region to a first depth within the semiconductor region. The method also includes forming gate trenches in the semiconductor region. The gate trenches may extend from the top surface of the semiconductor region to a second depth within the semiconductor region below the first depth. The method also includes forming a first plurality of silicon regions of a second conductivity type in the semiconductor region such that the first plurality of silicon regions form P-N junctions with the channel region along vertical walls of the first plurality of silicon regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 12/114,575, filed May 2,2008, which is a continuation of U.S. application Ser. No. 11/140,249,filed May 26, 2005, now U.S. Pat. No. 7,368,777, which claims thebenefit of U.S. Provisional Application No. 60/588,845, filed Jul. 15,2004, and U.S. Provisional Application No. 60/669,063, filed Apr. 6,2005, which are commonly assigned and incorporated herein by referencein their entirety for all purposes. Also, U.S. application Ser. No.11/140,249 is a continuation-in-part of U.S. patent application Ser. No.11/026,276, filed Dec. 29, 2004, now U.S. Pat. No. 7,345,342, which iscommonly assigned and incorporated herein by reference in its entiretyfor all purposes.

BACKGROUND

The present invention relates in general to the semiconductor technologyand more particularly to semiconductor accumulation-mode power deviceswith charge balance structures integrated therein.

FIG. 1 shows a cross section view of a conventional trenched-gateaccumulation-mode vertical MOSFET 100. MOSFET 100 is an n-channeltransistor with a gate terminal formed inside each of trenches 102,n-type regions 112 between trenches 102 (hereinafter referred to as thechannel regions), a drift region 106, and an n-type drain region 114.Unlike enhancement-mode transistors, accumulation-mode transistor 100does not include a blocking (p-type in this example) well or body regioninside which the conduction channel is formed. Instead a conductingchannel is formed when an accumulation layer is formed in channelregions 112. Transistor 100 is normally on or off depending on dopingconcentration of channel regions 112 and doping type of the gateelectrodes. It is turned off when channel regions 112 are entirelydepleted and lightly inverted. Because no inversion channel is formed,the channel resistance is eliminated thus improving the transistor powerhandling capability and its efficiency. Further, with no pn body diode,the losses in synchronous rectification circuits attributable to the pndiode are eliminated.

A drawback of accumulation transistor 100 is that drift region 106 needsto be lightly doped to support a high enough reverse bias voltage.However, a lightly doped drift region results in a lower on-resistanceand lower efficiency. Thus, there is a need for an accumulation-modetransistor with low on-resistance, high blocking capability, andimproved efficiency.

BRIEF SUMMARY

In accordance with an embodiment of the present invention, a method offorming an accumulation-mode field effect transistor includes forming achannel region of a first conductivity type in a semiconductor region ofthe first conductivity type. The channel region may extend from a topsurface of the semiconductor region to a first depth within thesemiconductor region. The method also includes forming gate trenches inthe semiconductor region. The gate trenches may extend from the topsurface of the semiconductor region to a second depth within thesemiconductor region below the first depth. The method also includesforming a first plurality of silicon regions of a second conductivitytype in the semiconductor region such that the first plurality ofsilicon regions form P-N junctions with the channel region alongvertical walls of the first plurality of silicon regions.

In one embodiment, the first plurality of silicon regions and the gatetrenches are alternately arranged so that each of the first plurality ofsilicon regions is between but laterally spaced from two adjacent gatetrenches.

In another embodiment, conduction channels are formed in the channelregion along the sidewalls of the gate trenches when theaccumulation-mode field effect transistor is in the on state.

In another embodiment, forming the first plurality of silicon regionsincludes forming body trenches in the semiconductor region, the bodytrenches extending from the top surface of the semiconductor region to adepth within the semiconductor region below the first depth, andsubstantially filling the body trenches with silicon material of thesecond conductivity type. The body trenches and the gate trenches may bealternately arranged so that each body trench is between but laterallyspaced from two adjacent gate trenches.

In another embodiment, the first plurality of silicon regions may beformed by implanting dopants of the second conductivity type into thesemiconductor region.

In another embodiment, the method also includes forming a gate electrodein each gate trench, the gate electrode being adjacent to but insulatedfrom the channel region.

In another embodiment, the semiconductor region includes an epitaxiallayer extending over and in contact with a substrate. The substrate andthe epitaxial layer may be the first conductivity type, and theepitaxial layer may have a lower doping concentration than thesubstrate. The channel region, the trenches and the first plurality ofsilicon regions may extend into the epitaxial layer.

In another embodiment, the method may also include forming a secondplurality of silicon regions of the first conductivity type in thechannel region such that each pair of the second plurality of siliconregions flanks sides of a corresponding one of the gate trenches.

The second plurality of silicon regions may have a higher dopingconcentration than that of the channel region. The method may alsoinclude forming a resistor coupled between the first plurality ofsilicon regions and the second plurality of silicon regions.

In another embodiment, the gate trenches and the first plurality ofsilicon regions are parallel-extending continuous stripes.

In another embodiment, the first plurality of silicon regions are formedin the semiconductor region such that an upper-most surface of each ofthe first plurality of silicon regions is below the top surface of thesemiconductor region.

In yet another embodiment, the method may also include forming columnsof silicon of the second conductivity type in the semiconductor regionso that the columns of silicon of the second conductivity type and thesemiconductor region together form alternating columns of first andsecond conductivity type silicon. No current may flow through thecolumns of silicon of the second conductivity type when theaccumulation-mode field effect transistor is in the on state.

In still another embodiment, each of the columns of silicon of thesecond conductivity type extend directly below a corresponding one ofthe plurality of first silicon regions.

These and other embodiments of the invention will be described withreference to the accompanying drawings and following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section view of a conventional trenched-gateaccumulation-mode vertical field effect transistor;

FIG. 2 shows a cross section view of an exemplary trenched-gateaccumulation-mode vertical field effect transistor with alternateconductivity type regions in accordance with an embodiment of theinvention;

FIGS. 3 and 4 show cross section views of two exemplary trenched-gateaccumulation-mode vertical field effect transistors with trenchedelectrodes in accordance with other embodiments of the invention;

FIGS. 5 and 6 are cross section views of two exemplary trenched-gateaccumulation-mode vertical field effect transistors withdielectric-filled trenches in accordance with other embodiments of theinvention;

FIGS. 7 and 8 show cross section views of two exemplary trenched-gateaccumulation-mode vertical field effect transistors with trenched diodesin accordance with other embodiments of the invention;

FIG. 9 shows a cross section view of a variation of the FIG. 3embodiment wherein the channel regions and the drift region are merged(i.e., have the same doping concentration) in accordance with anotherembodiment of the invention;

FIGS. 10 and 11 show cross section views of two variations of the FIG. 3embodiment wherein heavily doped opposite conductivity-type regions areformed in the channel regions in accordance with other embodiments ofthe invention;

FIGS. 12 and 13 show cross section views of two variations of the FIG. 3embodiment wherein heavily doped same conductivity-type regions areformed in the channel regions in accordance with other embodiments ofthe invention;

FIG. 14 shows a three dimensional view of the FIG. 13 embodiment;

FIG. 15 shows a cross section view of a variation of the FIG. 7embodiment wherein the channel regions are merged with the drift region(i.e., have the same doping concentration), and heavily doped oppositeconductivity-type regions are formed in the channel regions inaccordance with another embodiment of the invention;

FIG. 16 shows a cross section view of a variation of the FIG. 2embodiment wherein heavily doped same conductivity-type regions areformed in the channel regions in accordance with another embodiment ofthe invention;

FIG. 17 shows a cross section view of an exemplary planar-gateaccumulation-mode lateral field effect transistor with charge balancingelectrodes in accordance with another embodiment of the invention;

FIG. 18A shows a cross section view of an exemplary planar-gateaccumulation-mode lateral field effect transistor with trenched diodesin accordance with another embodiment of the invention;

FIG. 18B shows a top layout view of the embodiment shown in FIG. 18A;

FIG. 18C shows a cross section view along 18C-18C line in FIG. 18B;

FIG. 19 shows a cross-section view of a trenched-gate accumulation FETin accordance with an embodiment of the present invention;

FIG. 20 shows a variation of the FIG. 19 FET wherein a resistor 2718 iscoupled between the body and source regions;

FIG. 21 show yet another variation of the FIG. 19 FET wherein the bodyregions are buried in silicon;

FIG. 22 shows an exemplary implementation of the FIG. 19 FET in amulti-pillar charge balance structure; and

FIG. 23 shows an isometric view of a FET with charge balance structurewherein continuous stripes of body regions are used, in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION

In accordance with the present invention, charge spreading techniquesare advantageously integrated in vertical and lateral accumulation-modepower devices to achieve lower on-resistance, higher blocking capabilityand improved efficiency.

FIG. 2 shows a simplified cross section view of an exemplaryaccumulation-mode field effect transistor 200 with alternateconductivity regions 203, 205 extending vertically parallel to thecurrent flow. In this example, transistor 200 is an n-channel transistorwith a gate terminal formed inside each of trenches 202, n-type channelregions 212 between trenches 202, a drift region 206 that includesopposite polarity columnar n-type and p-type sections 203 and 205, andan n-type drain region 214. Unlike enhancement-mode transistors,accumulation-mode transistor 200 does not include a blocking (p-type inthis example) well or body region inside which the conduction channel isformed. Instead a conducting channel is formed when an accumulationlayer is formed in channel regions 212. Transistor 200 is normally on oroff depending on doping concentration of channel regions 212 and dopingtype of the gate electrodes. It is turned off when channel regions 212are entirely depleted and lightly inverted.

In the embodiments describe herein, those portions of the drift regionthrough which current flows when the transistor is in an on state arereferred to as the conduction regions.

The doping concentrations in opposite polarity regions 203 and 205 areadjusted to maximize charge spreading, which enables the transistor tosupport higher voltages and also results in more efficient use of thesilicon. The use of columnar opposite polarity regions parallel to thecurrent flow flattens the electric field distribution in the conductionregions by not allowing it to decrease linearly away from the junctionformed between regions 212 and 206. The charge spreading effect of thisstructure allows the use of a more highly doped drift region 206 whichadvantageously reduces transistor on-resistance. The dopingconcentration of the various regions may vary; for example, n-typeregions 212 and 203 may have the same or different dopingconcentrations. The embodiment wherein n-type regions 212 and 203 havethe same doping concentration is simpler to manufacture and thus lesscostly, and is attractive for use in low voltage applications. Thoseskilled in the art appreciate that an improved p-channel transistor canbe obtained by reversing the polarities of the various regions of thetransistor shown in FIG. 2.

An exemplary method of forming the accumulation-mode transistor in FIG.2 is as follows. An n-type epitaxial layer is formed over substrate 214in accordance with conventional methods. Using a masking layer, columnsof silicon are removed from the epitaxial layer using conventionalsilicon etching techniques. Silicon material 205 of p-type conductivityis then formed in the removed columns of silicon using known techniques.An n-type epitaxial layer 212 is then formed over epitaxial layer 206using conventional techniques. Trenched gate structures are then formedin epitaxial layer 212 using known methods.

FIG. 3 is a simplified cross section view of another accumulation-modefield effect transistor 300 with trenched electrodes for chargespreading purposes. Channel regions 312, drift region 306 and drainregion 314 are all of the same conductivity type which in this exampleis n-type. The doping concentration of channel regions 312 is adjustedto form a depleted blocking junction under no bias conditions. Insideeach trench 302, one or more buried electrodes 311 are formed under gateelectrode 310, all insulated from one another and from drift region 306by dielectric material 308. Buried electrodes 311 act as field platesand can be biased, if desired, to a potential that optimizes theircharge spreading effect. Since charge spreading can be controlled byindependently biasing buried electrodes 311, the maximum electric fieldcan be increased significantly. Different variations of this structureare possible. For example, the depth of trench 302 and the size andnumber of buried electrodes 311 can vary depending on the application.Charge spreading electrodes can be buried inside trenches that areseparate from active trenches that house the transistor gate electrode.An example of such an embodiment is shown in FIG. 4.

A method of forming the structure in FIG. 3 is as follows. An n-typeepitaxial layer is formed over substrate 314. An n-type layer 312 isformed along an upper portion of the epitaxial layer by implantingdopants into the epitaxial layer. Depending on the desired dopingconcentration, n-type dopants may be implanted into the epitaxial layerso that n-type layer 312 has a higher doping concentration than driftregion 306 of the epitaxial layer, or alternatively, p-typecounter-dopants may be implanted into the epitaxial layer so that n-typelayer 312 has a lower doping concentration than drift region 306 of theepitaxial layer. In another embodiment, regions 306 and 312 are formedby growing two n-type epitaxial layers having different dopingconcentrations. An exemplary set of process steps for forming thetrenched electrodes are described in the above-referenced commonlyassigned U.S. patent application Ser. No. 10/821,796 titled “SchottkyDiode Using Charge Balance Structure,” filed Apr. 9, 2004, incorporatedherein by reference in its entirety. The trenched-gate structures areformed using conventional techniques.

Another embodiment for an improved accumulation-mode transistor employsdielectric-filled trenches with an opposite polarity exterior liner.FIG. 5 is a simplified cross section view of an accumulation transistor500 according to this embodiment. Dielectric-filled trenches 520 extenddownward from the surface of the silicon well into drift region 506.Trenches 520 are substantially filled with dielectric material such assilicon dioxide. In this exemplary embodiment, transistor 500 is ann-channel transistor with trenched-gate structure. A p-type region 526lines the exterior sidewalls and bottom of dielectric-filled trenches520 as shown. Dielectric-filled trenches 520 reduce the outputcapacitance of the transistor while p-type liner 526 provides for chargebalancing in the drift region to increase the blocking capability of thetransistor. In one exemplary method of forming transistor 500, regions506 and 512 are formed in a similar manner to previous embodiments. Aset of process steps for forming the dielectric-filled trenches isdisclosed in the above-referenced commonly assigned U.S. Pat. No.6,376,878 to Christopher B. Kocon, titled “MOS-Gated Devices withAlternating Zones of Conductivity,” incorporated herein in its entirety.The trenched-gate structures are formed using conventional techniques.

In an alternative embodiment shown in FIG. 6, oppositely doped liners626N and 626P are formed adjacent the opposite sidewalls of adielectric-filled trench 620. That is, a dielectric-filled trench 620has a p-type liner 626P extending along the exterior sidewall on oneside, and an n-type liner 626N extending along the exterior sidewall onthe opposite side of the same trench. Other variations of thisaccumulation transistor with dielectric-filled trenches are possible.These include, for example, an accumulation transistor with a planar (asopposed to trenched) gate structure; an accumulation transistor with ap-type liner that covers only the exterior sidewalls and not the bottomof trenches 526; and an accumulation transistor with a single trenchstructure with a p-type liner that lines the lower portion of thetrench.

In another embodiment, an accumulation-mode transistor employs one ormore diodes stacked inside a trench for charge balancing purposes. Asimplified cross section view of an exemplary accumulation-modetransistor 700 according to this embodiment is shown in FIG. 7. In thisembodiment, the charge balancing diodes are formed inside the sametrench as the gate. Trench 702 includes gate electrode 710 below whichn-type 723 and p-type 725 silicon or doped polysilicon layers form pnjunctions. A dielectric layer 708 separates the diode structure fromgate terminal 702 as well as drift region 706. As shown, there is nodielectric layer along the bottom of trenches 720, thus allowing thebottom region 727 to be in electrical contact with the underlyingregion. In one embodiment, similar considerations to those dictating thedesign and manufacture of the gate oxide of MOS transistors are appliedin designing and forming insulating layer 708. For example, as in gateoxide of MOS transistors, the thickness of insulating layer 708 isdetermined by such factors as the voltage that insulating layer 708 isrequired to sustain and the extent to which the electric field in thetrench diode is to be induced in drift region 706 (i.e., the extent ofcoupling through the insulating layer). The charge spreading in thisembodiment is controlled by the optimization of the diode avalanchebreakdown and the width of the depletion in the trenched diodesdescribed more fully next.

As is well-known in this art, under reverse-bias conditions, theelectric field is highest at the junction between regions 706 and 712and linearly reduces in drift region 706 in the direction away from thejunction between regions 706 and 712 at a rate dictated by the dopingconcentration of regions 712 and 714. It is also well known that thelarger the area under the electric field curve (i.e., the lower the rateat which the electric field reduces), the greater is the breakdownvoltage. During operation, the diodes embedded in trenches 702 arereverse biased and thus the electric field is highest at the diodejunctions (e.g., at the junction between opposite conductivity typeregions 725 and 723). Through insulating layer 708, the electric fieldin the trench diodes induces a corresponding electric field in driftregion 706. The induced field is manifested in drift region 706 in theform of an up-swing spike and a general increase in the electric fieldcurve in drift region 706. This increase in the electric field resultsin a larger area under the electric field curve, which in turn resultsin a higher breakdown voltage.

It can be seen that by using multiple diodes in trenches 702, multiplespikes can be induced along the depth of conduction regions in driftregion 706. This results in an electric field curve which tapers downfrom its highest level at a far lower rate than in conventionalstructures. An almost trapezoidal-shaped area can thus be obtained underthe electric field curve in drift region 706 as opposed to theconventional triangular shape. A far greater breakdown voltage can thusbe obtained.

When structure 700 is biased in the conduction state, current passesthrough conduction region of drift region 706. By introducing diodetrenches 702, the electric filed across the reverse-biased trencheddiodes influences the charge distribution in adjacent drift region 706such that a more uniform charge spreading is obtained in drift region706. By spreading the charge more uniformly in drift regions 706, thesilicon area taken up by drift region 706 is more efficiently used.Hence, for the same size drift region 706, the portion of theon-resistance attributable to the conduction region of drift region 706is, in effect, reduced. This enables reducing the cell pitch for thesame on-resistance. Accordingly, diode trenches 706 enable optimizingstructure 700 to have higher breakdown voltage, lower on-resistance, andsmaller cell pitch than can be achieved by conventional techniques.

Other variations of accumulation-mode transistor 700 are possible. Onesuch variation is shown in FIG. 8. In this embodiment, diode trenches820 are disposed on either sides of gate trenches 802, extending wellinto drift region 806. Diode trenches 820 include one or more diodestructures made up of opposite conductivity type regions 823 and 825that form one or more pn junctions inside the trench. A thin dielectriclayer 821 extending along the trench inner sidewalls insulates thediodes in the trench from drift region 806. In another variation of FIG.8, not shown, one or more charge electrodes are formed in the gatetrench below the gate electrode.

An exemplary method of forming the structure in FIG. 8 is as follows. Ann-type epitaxial layer is formed over substrate 814. N-type region 812is formed in the epitaxial layer by implanting dopants into theepitaxial layer. Depending on the desired doping concentration, n-typedopants may be implanted into the epitaxial layer so that channelregions 812 have a higher doping concentration than portion 806 of theepitaxial layer, or alternatively, p-type counter-dopants may beimplanted into the epitaxial layer so that channel regions 812 have alower doping concentration than portion 806 of the epitaxial layer. Inanother embodiment, regions 806 and 812 are formed by growing two n-typeepitaxial layers having different doping concentrations. An exemplaryset of process steps for forming the trenched diodes are described inthe above-referenced commonly assigned patent application Ser. No.10/821,796 titled “Schottky Diode Using Charge Balance Structure,” filedApr. 9, 2004, incorporated herein by reference in its entirety. Thetrenched-gate structures are formed using conventional techniques.

FIGS. 2-8 illustrate accumulation transistor embodiments in whichvarious charge balancing structures are integrated in an optimum mannerto improve the transistor blocking capability, on-resistance, andcurrent density of the transistor. In FIGS. 9-14, the transistorembodiment with the trenched electrodes as shown in FIG. 3 is used toshow various structural modifications and features each with its owndistinct advantages. Although FIGS. 9-14 show these structuralmodifications in the context of the trenched electrodes transistorembodiment of FIG. 3, one or more of these structural modifications andfeatures can be combined with one or more of the various chargebalancing structures illustrated in FIGS. 2-8 depending on the desiredgoals and performance criteria.

In FIG. 9, the channel regions in which the conduction channel is formed(e.g., regions 312 in FIG. 3) are merged with the drift region to form asingle region 906 in which trenches 902 are formed. Thus, drift region906 extends from the top surface of the structure to substrate 914.Transistor 900 operates in the same manner as transistor 300 in FIG. 3.This embodiment has the advantage of a simpler manufacturing processbecause of elimination of the processing steps associated with formingthe channel regions between the trenches. To maintain the sametransistor current capability as the FIG. 3 structure, the dopingconcentration in region 906 needs to be increased. This in turn reducesthe transistor voltage blocking capability. But, due to the presence ofthe charge balancing trenched electrodes, the increased dopingconcentration does not result in as much of a reduction in thetransistor blocking capability. It is to be understood that the channelregions in the other embodiments illustrated in FIGS. 4-8 may similarlybe merged with the drift region.

In FIG. 10, heavily doped p-type regions 1018 are formed in channelregions 1012 between trenches 1002. P-type regions 1018 minimize thearea of channel regions 1012 and reduce device leakage. P-type regions1018 also allow for p+ contact which improve hole current flow inavalanche and improve device robustness. Each of p+ regions 1018 may bea single continuous region extending the length of the mesa (i.e., inthe dimension perpendicular to the page) between trenches, or may be oneof multiple p+ regions formed at intervals along the length of the mesabetween the trenches. FIG. 11 shows a three-dimensional view of avariation of the structure in FIG. 10. As shown, p+ regions 1118laterally extend to abut the trenches between which they are located.However, p+ regions 1118 are formed at intervals along the length of themesa so that channel regions 1112 can be contacted along their topsurface in order to accommodate current flow when the transistor isturned on. An advantage of extending the p+ region between adjacenttrenches as compared to FIG. 10 wherein p+ regions 1018 are laterallyspaced from the trenches is that the alignment tolerance required by thespacing between the p+ regions and the edge of trenches in FIG. 10 iseliminated thus resulting in a smaller cell pitch. The p+ regions can beused in a similar manner in any one of the accumulation transistorsdescribed herein.

In FIG. 12, n-type channel regions 1212 include more heavily dopedshallow n+ source regions 1203. Heavily doped source regions 1203 resultin better contact with the overlying source electrode, and thus reducethe contact resistance. Heavily doped source regions 1203 can extendalong the top surface of channel regions 1212 between trenches as shownin FIG. 12, or can alternatively be formed as two regions adjacenttrench sidewalls along the top edge of channel regions 1212 as shown inFIG. 13. An advantage of this approach is the source metal contactinglightly doped channel regions 1212 forms a Schottky contact therebetweenwhich helps minimize leakage current. FIG. 14 shows a 3-dimensional viewof the structure illustrated in FIG. 13. As shown, n+ source regions1432 are formed at intervals along the length of the mesa betweentrenches in channel regions 1412. N+ regions 1432 may alternatively be asingle continuous region extending the length of the mesa betweentrenches. The heavily doped source regions as illustrated in FIGS. 12-14can be used in the same manner in any one of the accumulationtransistors described herein. The heavily doped regions in FIGS. 10-14can be formed in respective channel regions using known masking andimplanting techniques.

It is to be understood that any of the charge balancing structuresillustrated in FIGS. 2-8 may be combined with one or more of thestructural variations and features illustrated in FIGS. 9-14 as desired.For example, FIG. 15 shows the charge balancing structure illustrated inFIG. 7 in combination with the highly-doped p+ regions, wherein thechannel regions are merged with the drift region to form a singlesilicon region 1506 in which the trenches are formed. FIG. 16 showsanother example wherein the charge balancing structure illustrated inFIG. 2 is combined with the highly doped n+ regions.

Also, variations on exemplary vertical MOS-gated accumulationtransistors have been discussed to illustrate the various features andadvantages of the present invention. One of skill in the art appreciatesthat these can be implemented in other types of devices includingplanar-gate vertical transistors, planar-gate lateral transistors,trenched-gate lateral transistors, diodes, bipolar transistors and thelike. FIGS. 17 and 18A-18C show few examples of such devices. In FIG.17, laterally-positioned electrodes 1711 a and 1711 b are integrated ina planar-gate accumulation-mode lateral transistor 1700. In FIGS.18A-18C, laterally-extending diode trenches 1820 are integrated in aplanar-gate accumulation-mode lateral transistor 1800. FIG. 18B shows atop view of transistor 1800. FIGS. 18A and 18C respectively show thecross section views along lines 18A-18A and 18C-18C in FIG. 18B. In theFIGS. 17 and 18A-18C embodiments, as in the previous embodiment, thecharge balance electrodes and diodes extend parallel to the currentflow.

FIG. 19 shows a cross-section view of a trenched-gate accumulation FET1900 in accordance with an embodiment of the present invention. FET 1900includes an n-type substrate 1902, an n-type drift region 1904, and ann-type mesa region 1906. Gate trenches 1908 extend through mesa region1906 and terminate in drift region 1904. Each gate trench 1908 is linedwith a dielectric layer 1910 along its sidewalls and bottom, andincludes a recessed gate 1912. N-type source regions 1914 flank eachside of trenches 1908 as shown.

A p-type body region 1916 extends into mesa region 1906 between eachpair of gate trenches 1908. Body regions 1916 form a p-n junction withmesa region 1906. The p-n junction results in a stronger pinch-off forimproved blocking and Rdson. The amount of pinch-off, determined by thevoltage and charge in the pinched-off region, influences the blockingcharacteristics. Also, the ratio of the pinched channel length (Lp inFIG. 19) to the pinched channel width (Wp in FIG. 19) needs to be keptas high as possible (preferably >1) for good blocking capability. Notethat the pinched channel length Lp is the spacing between the bottom ofregion 1914 and the bottom of the shallower one of the gate 1912 andbody region 1916.

In FIG. 19, body regions 1916 extend deeper than mesa region 1906,however, in another embodiment, a shallower body region (extending to ashallow depth within region 1906 or to a depth just above the lowersurface of mesa region 1906) may be formed. But, a deeper body region,as shown, is advantageous since the built-in potential of the p-njunction (Vbi,max value is Eg/q, ˜1.1V) also pinches the channel thusresulting in better blocking capability. In other embodiments, each bodyregion 1916 is advantageously formed as a continuous stripe along thedimension perpendicular to the page. Using a continuous stripe, asopposed to multiple body regions with gaps therebetween, prevents thepossibility of leakage current to flow through the gaps. FIG. 23 moreclearly illustrates an implementation of continuous body regions 2316 inone embodiment of the invention. In an alternate variation, thecontinuous body region is broken up into multiple regions, however thegaps between the body regions needs to be kept small enough so that thegaps can be fully depleted by the built-in potential. For cellulardesigns, each body region 1916 (FIG. 19) is enclosed within the cellsuch that it helps in pinching off the entire channel.

Incorporation of the body regions in the accumulation FET, in accordancewith the different embodiments of the invention, results in a netbuilt-in voltage which is about four times greater than that without thebody regions. Hence the charge (Q) in the channel can be kept higher toobtain better on-resistance with high blocking capability. Since theaccumulation layer mobility is higher than inversion layer mobility, thechannel resistance is lower for accumulation FETs. This is particularlytrue for silicon carbide (SiC) based devices. Also, the pinched regions(i.e., the portions of mesa region 1906 between body region 1916 and itsadjacent gate trenches 1908) can carry current, particularly at highercurrent densities, in parallel to the channel regions. This furtherlowers the Rdson thus improving device power handling capability and itsefficiency. Also, due to higher transconductance and lower Vth, theaccumulation FET in accordance with the invention is particularlyattractive for low gate voltage, high current applications.

Conventional accumulation FETs rely solely on MOS depletion and henceare sensitive to oxide charge variations and work function differences.However, in the accumulation FET of the present invention, majority ofthe blocking is provided by the p-n junction, and thus the FET device isless sensitive to oxide charges. The threshold voltage (Vth) of theaccumulation FET is primarily controlled by Wp (FIG. 19) and the dopingconcentration of mesa region 1906. In contrast, for conventionalenhancement mode FETs, Vth is controlled by the Tox, Qox, p-welldose/drive, source junction depth (Xjs), and p-well junction depth(Xjw). Therefore, the Vth of accumulation FET of the present inventioncould be better controlled, particularly for low Vth devices.

The p-n diode formed between body region 1916 and mesa region 1906injects minority carriers when forward biased and thus the associatedreverse recovery may be undesirable in certain applications. Thisundesirable effect can be eliminated or substantially minimized in anumber of ways. In a first approach, the doping concentration of bodyregion 1916 is reduced. In a second approach illustrated in FIG. 20, aresistor 2018 is coupled between body regions 2016 and source regions2014. Resistor 2018 drastically reduces the diode current, but stillkeeps body regions 2016 at ground potential during normal operation. Thecapacitance associated with the p-n junction needs to be charged throughresistor 2018 during switching from on to off state. Thus, theresistance value of resistor 2018 should be carefully chosen such thatthe switching is not adversely affected while the injection of minoritycarriers is minimized. A third approach is to float body regions 2016either by not biasing them to any potential, or as illustrated in FIG.21, burying them in silicon. The latter approach completely eliminatesthe body diode problem.

FIG. 22 shows an exemplary implementation of an embodiment of theinvention in a multi-pillar charge balance structure. In general,integrating super junction structures, such as that in FIG. 22, reducesthe Rdson drastically. FIG. 23 shows an isometric view of an exemplarystripe geometry device with continuous body stripes 2316 andmulti-pillar charge balance structure.

The table below compares simulation results for a 30V accumulation modeFET in accordance with an embodiment of the invention and a 30Vconventional enhancement mode FET. No charge balance structure was usedin either device. Simulation parameters used include: trench width=mesawidth=0.5 μm; Tox=250 Å; trench depth=1 μm; p-well/p-body depth=0.8 μm;Epi thickness=2 μm; Epi doping=2×10¹⁶/cm³; doping of mesa region=1×10¹³/cm³; BV is measured at J=1 mA/cm²; Rdson at Vg=5V, J=100 A/cm²;Vth at J=10 mA/cm²; and Qgt at Vg=5V.

Rsp; Body Gm @Vg = 5 V, Vth BV diode @Vdd = 2 V, Qgtot 100 A/cm² @10mA/cm² @1 mA/cm² Irrm J = 100 A/cm² @5 V (μΩ-cm²) (V) (V) (A) (S/cm²)(nC/cm²) Accumulation 47.2 0.74 32.2 1.4 2427 1620 FET Enhancement 51.10.74 34.7 3.7 1676 1811 FET

As can be seen, the accumulation FET has lower Rsp, Qg and Irrm, and amuch higher transconductance (Gm) than the conventional enhancement FET.Also, simulation results showed that the electric field lines in thedrift region of the accumulation FET had little crowding, were equallyspaced and substantially flat, and as such similar blocking capabilityas the conventional FET was obtained.

Known process techniques can be used to form the accumulation FET of thepresent invention. For example, the manufacturing methods describedabove may be used and modified in one of a number of ways to obtain thestructures shown in FIGS. 19-23. Body regions 1916 (FIG. 19) can beformed in a number of ways. In a first approach, multiple implants ofp-type dopants into mesa region 1906 are carried out using varyingenergies to form body regions 1916 with a relatively uniform dopingconcentration and a rectangular profile with minimal lateral bulge. In asecond approach, a trench may be formed through mesa regions 1906between adjacent gate trenches 1908, and then selectively filled withp-type silicon using well know selective epitaxial growth (SEG)techniques. The latter approach provides good aspect ratio with arectangular shape, and uniform doping concentration. These twotechniques can be modified, as known in this art, to form the buriedbody regions 2120 in FIG. 21. Resistor 2018 in FIG. 20 may be formed ina number of ways. For example, a polysilicon thin film resistor may beused or alternatively the high metal contact resistance to lightly dopedp-type body may be used as the resistor.

FIGS. 19-23 show the gate trenches terminating in the drift region,however, the gate trenches can also extend through the drift region andterminate within the substrate. While FIG. 23 shows an open cellgeometry, the various embodiments of the invention may be implemented inany closed cell geometry, including hexagonal and square shapegeometries. Also, while the various embodiments illustrated in FIGS.19-23 are implemented in conventional silicon, these embodiments andtheir obvious variants can also be implemented in silicon carbide,gallium arsenide, gallium nitride, diamond or other semiconductormaterials.

The cross-section views of the various embodiments described herein maynot be to scale, and as such are not intended to limit the possiblevariations in the layout design of the corresponding structures. Also,the various transistors can be formed in stripe or cellular architectureincluding hexagonal or square shaped transistor cells

Although a number of specific embodiments are shown and described above,embodiments of the invention are not limited thereto. For example, it isunderstood that the doping polarities of the structures shown anddescribed could be reversed and/or the doping concentrations of thevarious elements could be altered without departing from the invention.As another example, the various exemplary accumulation-mode verticaltransistors described above have the trenches terminating in the driftregions, but they can also terminate in the more heavily dopedsubstrate. As yet another example, the above-described embodiments areformed in silicon, but they could also be formed in silicon carbide,GaAs or other types of semiconductor materials. Also, the features ofone or more embodiments of the invention may be combined with one ormore features of other embodiments of the invention without departingfrom the scope of the invention.

Therefore, the scope of the present invention should be determined notwith reference to the above description but should, instead, bedetermined with reference to the appended claim, along with their fullscope of equivalents.

What is claimed is:
 1. A method of forming an accumulation-mode fieldeffect transistor, the method comprising: forming a channel region of afirst conductivity type in a semiconductor region of the firstconductivity type, the channel region extending from a top surface ofthe semiconductor region to a first depth within the semiconductorregion; forming a gate trench in the semiconductor region, the gatetrench extending from the top surface of the semiconductor region to asecond depth within the semiconductor region below the first depth; andforming a silicon region of a second conductivity type in thesemiconductor region and extending to a third depth of at least thesecond depth such that the silicon region forms a P-N junction with thechannel region along a wall of the silicon region.
 2. The method ofclaim 1 wherein the silicon region and the gate trench are arranged sothat the silicon region is laterally spaced from the gate trench.
 3. Themethod of claim 1 wherein a conduction channel is formed in the channelregion along a sidewall of the gate trench when the accumulation-modefield effect transistor is in an on state.
 4. The method of claim 1wherein the forming the silicon region includes: forming a body trenchin the semiconductor region, the body trench extending from the topsurface of the semiconductor region to the third depth within thesemiconductor region below the second depth; and substantially fillingthe body trench with silicon material of the second conductivity type.5. The method of claim 1 wherein the silicon region is formed byimplanting a dopant of the second conductivity type into thesemiconductor region.
 6. The method of claim 1 further comprising:forming a gate electrode in the gate trench, the gate electrode beingadjacent to but insulated from the channel region.
 7. The method ofclaim 1 wherein the semiconductor region includes an epitaxial layerextending over and in contact with a substrate, the substrate and theepitaxial layer being of the first conductivity type, the epitaxiallayer having a lower doping concentration than the substrate, and thechannel region, the gate trench, and the silicon region extending intothe epitaxial layer.
 8. The method of claim 1 further comprising:forming a source region of the first conductivity type in the channelregion such that the source region flanks a side of the gate trench, thesource region having a higher doping concentration than a dopingconcentration of the channel region.
 9. The method of claim 8 furthercomprising: forming a resistor coupled between the silicon region andthe source region.
 10. The method of claim 1 wherein a plurality of gatetrenches including the gate trench and a plurality of silicon regionsincluding the silicon region are disposed in parallel-extendingcontinuous stripes.
 11. The method of claim 1 wherein the silicon regionis formed in the semiconductor region such that a top surface of thesilicon region is below the top surface of the semiconductor region. 12.The method of claim 1 further comprising: forming a column of silicon ofthe second conductivity type in the semiconductor region of the firstconductivity type so that the column of silicon of the secondconductivity type and the semiconductor region of the first conductivitytype together form at least part of alternating columns of first andsecond conductivity type silicon, substantially no current flows throughthe columns of silicon of the second conductivity type when theaccumulation-mode field effect transistor is in an on state.
 13. Themethod of claim 12 wherein the column of silicon of the secondconductivity type extends directly below a corresponding silicon regionof the second conductivity type.
 14. The method of claim 1 wherein theforming the silicon region of the second conductivity type includesforming the silicon region of the second conductivity type having a topsurface that is coplanar with the top surface of the semiconductorregion of the first conductivity type.